Field Value for Custom Components


Is there a list of abbreviations one should use for the Field Value of custom components? The default seems to be U. It seems:

R: Resistors
J: Connectors
U: Logic chips/processors/etc.


You have a choice of several standards, a popular one is IPC-2612, a summary is published in

Most are mnemonic, I always wondered why U for ICs, some people use “IC” which is more obvious.


Integrated Circuit maybe for IC?




I forget where I found this long ago, but here is an older standard (drafted 1975, reaffirmed 1993) that has both component symbols and reference designators. See PDF page 231 (document page 211) for the start of the section on reference designators (called Class Designation Letters in the document). See also PDF page 261 cross-referencing this document (Y32.2) to IEC 113-2. (Even the standards don’t agree, as usual.)

IEEE Std 315-1975 & ANSI Y32.2-1975 - electric-symbols.pdf (3.2 MB)

It is very detailed (for example showing what symbols to use with switches to indicate what activates them; thermal, temperature, push, pull, etc) but possibly outdated. But even being outdated if you read between the lines you can some historical context for an understanding of why things are they way they are today.

For example an IC having U as a designator prefix is because it is considered an assembly (some one built it), but it isn’t something that can then be disassembled. Thus “inseparable assembly”. One could argue that vacuum tubes are similar (because they are), but my theory has two parts: (remember this is IMHO, and I’m not a historian…)

  • First, ICs were late comers to the scene, so instead of making a new designator for something that had yet to prove itself in the industry, it was just shuffled into an existing category that it kind of fits into.
  • Second, the name “integrated circuit” alludes to a separate entity. It’s a full circuit unto itself so could be considered a separate assembly, thus either A or U, but again because it isn’t able to be disassembled that would make it U.


That still doesn’t explain why U stands for “inseparable assembly”… I guess there were no other free letters, like M for Module.

Similarly Q for transistor, T was already used for something else.

It’s pretty arbitrary anyway, at the end of the day you just need a unique reference, I don’t think anything ever depends on R meaning resistor. To a pick and place machine, they are all just objects. Obviously it is confusing to people if you used some other letter for resistors, but on one project we ran out of reference designators for resistors (only 3 digits allowed) so we had to use another letter in addition…


Thanks everybody for the input. This was very helpful and enlightening. That list on Wikipedia will probably be my reference. My google searches didn’t bring up much but now I know what term to search for, i.e. reference designator.


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